When interlaced television signals are "up-converted" to a non-interlaced or "progressive scan" form to double the number of lines for display, the displayed images may tend to exhibit unsharp transitions in vertical direction, This lack of sharpness is normally due to the use of vertical interpolation or line averaging to create "extra" lines for display. The result of that kind of up-conversion is very often that the original interlace picture looks sharper than the up-converted one. In addition to that, large format color TV tubes generally produce a high peak brightness level in order to be acceptable in the consumer market. This presents another problem in that a large beam current in bright areas can lead to a subsequent de-focusing of the resulting spot on the display.
To overcome these disadvantages, one may use a method of sharpness enhancement commonly known as vertical scan velocity modulation (hereafter, VSM). In systems employing vertical scan modulation the interpolated line (which represents along horizontal structures the center of the transition) is deflected away from the transition region towards the adjacent line carrying the higher luminance level. This results in an improved sharpness due to an enhanced overshoot of the luminance on the screen without increasing the beam current and avoids the problem of enlarged spot size (blooming) that is one of the potential drawbacks of conventional video signal peaking techniques.